The present invention relates to a structure for transporting and installing or retrieving underwater equipment.
The invention also relates to a method for transporting and installing or retrieving underwater equipment using such a structure.
In the field of underwater deposit mining, such as oil or gas mining, for example, it is common to place equipment, such as distribution boxes, manifolds, or pumps or separators, on the sea bottom.
This type of heavy equipment is most often put in place using hoisting cranes arranged on handling vessels or offshore mining vessels.
To that end, the underwater equipment to be installed is brought close to the mining site by a floating barge or a transport vessel.
Using a crane carried by another vessel or barge, the equipment is lifted and lowered by the crane provided with a cable, then placed on the sea bottom.
This method, which is the method most commonly used, has drawbacks.
The first drawback lies in the capacity of the hoisting cranes, which in some cases requires the addition of controllable buoyancy modules under the underwater equipment to be placed on the sea bottom in order to reduce the tension on the lowering cable.
The second drawback lies in the fact that during lifting of the equipment from the vessel or the transport barge, the equipment is suspended from a cable above the water level. When the equipment is lowered towards the bottom of the water, it will pass below the level of the water.
If the surface of the water is choppy, the equipment will undergo significant movements when the water level is broken, and these movements will be directly passed on to the cable supported by the crane.
These movements cause successive tensions and releases in said cable, as well as in the support means for the cable.
Also known is a method described in document U.S. Pat. No. 7,011,473 that consists of towing the underwater equipment to its installation site below the surface of the water with the aid of a vessel and maintaining the equipment using a buoy situated on the surface of the water.
However, towing the underwater equipment below the surface of the water poses safety problems and the equipment is suspended from a buoy when it is towed, said buoy being subjected to the various motions of the swell on the surface of the water.
Another method described in application US 2008/0035327 consists of placing the underwater equipment in a support frame and fastening said frame carrying the underwater equipment to the bottom of a transport barge.
At the installation site, the support frame and underwater equipment assembly is lowered using a drawworks, and said equipment is positioned on the sea bottom and disconnected from the support frame. Then, the support frame is raised below the hull of the barge.
The main drawback of this method lies in the towing of the underwater equipment below the water level to the installation site.